Light Path Node and Mix Shader
The factor of the Mix Shader node is the fraction of the second shader that contributes to the output shader.  If the factor is 0.8 then the output shader will be 80% the second input and 20% the first input.

The Is Camera Ray output (along with most of the other Light Path outputs) is a boolean (true = 1, false = 0).  So if the ray is a camera ray (true), the value going to the mix factor will be 1 and the output will be 100% the second shader input.